The present invention relates to an apparatus for emitting radiation. In particular, the present invention relates to tuning resonant structures, such as cavities in a linear accelerator.
Linear accelerators may be used for various purposes. For example, linear accelerators may be used in scientific research or in the medical environment. Although linear accelerators may be used for a variety of applications, the examples herein are described in conjunction with the medical environment for exemplary purposes. In medical applications, a beam of charged particles, such as electrons, from a linear accelerator may be directed at a target which is made of a material having a high atomic number to produce an X-ray beam for radiation therapy. In other applications, protons or heavy ions may be used instead of electrons. Alternatively, the beam of charged particles may be applied directly to a patient during a radio-surgical procedure. Such radio-surgery has become a well established therapy in the treatment of brain tumors. A high energy beam may be directed at a localized region to cause a breakdown of one or both strands of the DNA molecule inside cancer cells, with the goal of at least retarding further growth and preferably providing a curative cancer treatment.
A conventional linear accelerator (xe2x80x9clinacxe2x80x9d) typically includes a series of accelerating cavities that are aligned along a beam axis. A particle source, typically an electron gun, directs charged particles into the first accelerator cavity. As the charged particles travel through the succession of accelerating cavities, the particles are focussed and accelerated by means of an electromagnetic field. For example, a radio frequency (RF) source may be coupled to the accelerator to generate the necessary field to operate the linac. Often, the output beam is directed to a magnetic bending system that functions as an energy filter. In the medical environment, either an output beam of high energy particles or an X-ray beam generated by impinging a target with the output beam is then employed for radiation treatment of a patient.
Optimal performance of the accelerator typically requires a match between the resonant frequency of the cavity structure and the frequency of the driving signal. In order to determine the resonant frequency of a cavity structure, such as a side-coupled structure, each cavity or cell of the structure is typically tuned to a specific cell frequency which may be different from a resonant frequency of the structure. The frequencies of the various cells in the structure may be combined to result in the resonant frequency of the structure. Once the resonant frequency of each cell is determined, the cavity of a cell may be deformed to match a predetermined required frequency of each cell.
Typically, a linear accelerator, such as a linear accelerator used in a medical environment, includes multiple access holes in the structure. These access holes may include a beam hole through a set of main cells which may be used for facilitating passage of a beam of charged particles and also for facilitating measurement of the frequency of cells. Access holes may also include side holes which may also be used for the purpose of measuring the frequency of cells. Two connections, one to convey an excite signal and the other to convey a pickup signal, are typically inserted into these holes for the purposes of radiating a signal to excite a particular cell and measuring a resonant frequency of that cell. The two connections are typically inserted into access holes. For S band frequencies, an example of a size of such a beam hole is an inside diameter of approximately 10 mm, while an example of a size of a side access hole is an inside diameter of approximately 8 mm. For X band frequencies, an example of a size of a beam hole is approximately 3-4 mm. For further background information regarding the tuning of a resonant cavity, LOS ALAMOS MESON PROTON FACILITY (LAMP) 805 MHZ ACCELERATOR STRUCTURE TUNING AND ITS RELATION TO FABRICATION AND INSTILATION, by G. R. Swan, IEEE NS-16, 1965 may be referred.
Once tuning is completed, the side access holes in the cavity structures are typically sealed in order to establish and maintain a vacuum inside the cavity structures. It would be desirable to be able to tune a structure which is simpler to manufacture by minimizing the holes typically used for tuning. An example of such a structure is disclosed in U.S. Pat. No. 5,734,168, entitled MONOLITHIC STRUCTURE WITH INTERNAL COOLING FOR MEDICAL LINAC, issued to Yao on Mar. 31, 1998. Despite the fact that the holes in such a structure, or in a similar structure, are minimized, it is still typically necessary to tune such a structure. Additionally, tuning a structure designed for X band frequencies may be difficult due to the need for fitting two connections into such a small beam hole. The present invention addresses such a need.
An embodiment of the present invention includes the use of a single connection to measure the resonance frequency of main cells or side cells of a resonant structure, such as a linear accelerator cavity structure. Only one antenna is required to perform both the tasks of exciting a cavity structure and picking up the resonant frequency signal. According to an embodiment of the present invention, a first antenna probe is inserted into the main cells of a linear accelerator cavity structure. The first antenna probe includes an antenna window which may be positioned approximately in the center of a main cell adjacent to a target side cell in order to measure the resonance frequency of a target side cell. All non-target side cells adjacent to the main cell aligned with the antenna window are then shorted. For example, the non-target cells may be shorted by metal surrounding the first antenna probe at locations other than the antenna window. A signal is sent and a resonance frequency is noted. In order to measure the resonance frequency of a target main cell according to an embodiment of the present invention, a second antenna probe is inserted into a side cell adjacent to the target main cell. The main cells adjacent to the target main cell are then shorted and side cells adjacent to the target main cell are also shorted. A signal is sent and the resonance frequency of the target main cell is then measured.
A method according to an embodiment of the present invention for determining a resonance frequency of a resonant structure is presented. Such a method comprises steps of providing an excite signal in a first portion of a resonant structure, the excite signal being provided by a probe, wherein the probe includes a single signal wire. A reflected signal is received through the probe, and a resonance frequency of a second portion of the resonant structure is determined.
In another aspect of the invention, a system according to an embodiment of the present invention for determining a resonance frequency of a resonant structure is also presented. The system comprises a first portion of a resonant structure; a second portion of the resonant structure; and a probe. The probe includes a single signal wire, and the probe is configured to provide an excite signal in the first portion of the resonant structure. The probe is also configured to receive a reflected signal related to a resonance frequency of the second portion of the resonant structure.